This invention relates to fiber optic transmission. In particular, the invention relates to a method of providing supervisory signals to optical amplifiers in an optical transmission line. This is of particular relevance to fiber optic transmission systems used for long distance transmission, for example, in undersea cables used for intercontinental signal transmission.
An example of a fiber optic transmission system is illustrated schematically in FIG. 1. The system comprises a transmission fiber (5) carrying signals at eight carrier wavelengths (6). The transmission fiber extends from a first main station, or send station, (1) to a second main station, or receive station, (2). Spaced along the transmission fiber (5) are add/drop multiplexers (4). At each add/drop multiplexer (4) the signal at a selected one of the eight carrier wavelengths is dropped to a minor station (3), and replaced with a new signal at the same carrier wavelength from the minor station (3).
To maintain signal strength on a long transmission line, it is necessary to amplify the signal at periodic intervals. It is possible to do this electrically, by conversion of the optical signal on the fiber to an electrical signal followed by amplification of the electrical signal and then conversion back to an optical signal. However, it is generally preferred to use optical amplification methods, which have the advantage that there is no need to convert the transmitted optical signal to an electrical signal until it needs to be processed at a receiving station. In the illustrative example depicted in FIG. 1, there are 200 optical amplifiers between the main stations (1), (2), the add/drop multiplexers being spaced at forty amplifier intervals.
The structure of a typical optical amplifier unit (or repeater) is illustrated in FIG. 2. The input fiber (5a) of the fiber transmission line (5) carries the input signal, which first enters a fiber optic coupler (14a). Here the signal is split into two unequal parts (typically in a ratio of 1:20), the smaller part of the coupler output being branched off to a monitor diode (17a). The main output of the coupler (14a) passes through into an erbium doped fiber amplifier (EDFA) (11). The amplified output from the EDFA (11) passes into a first input (19a) of a multiplexer (16), which is adapted such that substantially the entire signal at the carrier wavelength passes out through a first output (19b).
In this example the carrier wavelength are selected to lie close to 1560 nm. An EDFA provides amplification for light at 1560 nm when it is pumped with light at 1480 nm. The pump light is provided by an appropriate pump laser (12), which may be, for example, an InGaAsP laser. Pumping light is provided through second port (19c) of the multiplexer (16), the multiplexer being adapted such that substantially all the pumping light is transmitted to the EDFA through first input (19a) (serving as an output in this direction).
The multiplexer (16) is adapted such that at the carrier wavelengths, substantially all the light is transmitted directly from first multiplexer input (19a) to the first output (19b) with no transfer across the coupler to the second port (19c), whereas at the pumping wavelength, there is a substantially complete transfer of pumping light across the coupler from second port (19c) to the first input (19a). It is possible through alternative structures to pump the EDFA at the input, or in the middle, rather than at the output side. It is possible to pump, for example, at both input and output sidesxe2x80x94this can be advantageous in the case of failure in one pumping connection.
As the EDFA amplifies in both directions, an isolator (15) is required in the output path of the carrier wavelength signal to prevent instability and interference, which could otherwise result from reflections at the output port of the repeater. The isolator (15) has very low attenuation in the forward direction (typically  less than 1 dB) and very high attenuation in the reverse direction (typically  greater than 25 dB). The output from the isolator (15) is provided as input to a further coupler (14b), from which the greater part of the signal is transmitted back out onto the transmission fiber (5) at output (5b), a small part of the signal being split off to a further monitor diode (17b).
Monitor diodes (17a), (17b) hence enable monitoring of the input and output power levels of the EDFA. The input power level monitor (17a) monitors the light received from the previous amplifier via the cable. Any fault in the cable or in the previous amplifier will change the input power. As the previous amplifier has had its output power level monitored (by a monitor diode (17b)), the fault can be located to the previous repeater or to the cable in between. Either monitor diode may also be used as a means of receiving supervisory signals for the supervisory section (13): advantageously, the input monitor diode (17a) is used for this purpose, as if the repeater itself is faulty the output monitor diode (17b) may not be able to detect the supervisory signals. Separate photodiodes (not shown) are provided so that the power of the pump laser (12) can be monitored at the supervisory section (13).
It is a necessary feature of such extended fiber optic systems that the repeater units can be subjected to external control: for example, to adjust the gain of the amplifiers in order to optimise a given signal or correct an imbalance. For such adjustment to be possible, it is also necessary for signals indicative of the status of the repeater to be fed back and/or forward to an external control point. This is also desirable in the event of a physical break or other fault in the cable: the position of the break or fault in terms of the repeaters on the cable can be determined by use of such feedback. Normally, such responses signals are sent back to the terminal that initiated the response. This can be achieved, for example, with a FIG. 2 repeater by modulation of the pump laser power by supervisory section (13). This will result in modulation of the output power of the traffic signals output by the EDFA, this modulation being detectable at a receiving station.
An important criterion in design of a system to transmit supervisory information between external control point and repeaters is the simplicity of the resulting repeater structure, and another is the minimization of the effect (e.g. noise) on the traffic signals themselves. High levels of reliability are also important, especially where failure necessitates the repair of an undersea cable. As the nature of the external control point (at a terminal station, where signal generation is straightforward and sophisticated signal processing possible) and of the repeater (where simplicity and reliability are of particular importance) are very different, different solutions are required to satisfy the same criteria. It is therefore preferable to use different approaches to send information from the repeaters to the external control point from those used to send supervisory information or instructions from the external control point to the repeaters.
Provision of information from the repeater to the external control point is generally achieved by a passive loop-back system. An exemplary arrangement is described in Hirst et al, Electronics Letters 29 (3): 255-6, 1993. For the signal travelling in one direction along the trunk of a fiber optic cable system, an optical coupler is provided at the output side of the amplifier, and a small part of the entire signal amplified is split off. The split-off signal is then looped back by means of a further optical coupler on to the fiber carrying signals along the trunk in the other direction. Typically, each coupler will provide about 10 dB of loss for this loop-back path, with the result that approximately 1% of the signal amplified is looped back. The transmitted test signal is thus received back at the external control station on another fiber (and much attenuated) for processing. By averaging and correlation techniques, such looped test signals can be discriminated from noise. By comparison of looped signals from different repeaters with each other, it is possible to deduce which repeater has an especially low or high gain, or to determine in which section of cable there is a cable break.
The present invention is directed to the different problem of provision of supervisory information to the individual repeaters from the point of external control. Various options for solution are discussed in Hadjifotiou, A., Brannan, J., Hirst, I. J., xe2x80x9cSupervisory Options for Optical Amplifier Systemsxe2x80x9d, Fourth IEE Conference on Telecommunications Systems in Manchester, England, IEE Conference publication 371, Apr. 18th to 21st 1993, pp 53 to 56. Several of the options there considered were found to have significant disadvantages. One option proposed was to transmit supervisory signals along the electrical power feed to the fiber optic cable: this, however, offers only a low bandwidth for communication and in long systems may lead to a need for high voltage components in order to satisfy power feed requirements. An alternative option is to use a stand alone system with a wholly separate channel to that used for data transmission, for example a 1300 nm channel, used for its operation. Such an arrangement offers a very high capacity, but requires the use of substantial additional hardware. This is because the 1300 nm signal will not be amplified by the amplifiers for the traffic signals and thus will not be regenerated without a separate regeneration network: a 1300 nm laser or LED and a suitable optical receiver are thus required at each repeater.
A preferred option is to employ modulation of the optical carriers used for transmitting data signals. The arrangement shown in FIG. 2 is in fact adapted for this form of supervisory control. The principle of operation is that low frequency, low intensity supervisory information is modulated onto the optical data signal sent out from the terminal providing the external control. This is detected at the repeater by a low frequency receiver: in the FIG. 2 case, the low frequency receiver for extracting supervisory information is a photodiode (17a).
A photodiode is a semiconductor device adapted to produce an output current proportional to the power of the light incident upon it. A silicon diode is an advantageous choice of photodiode for use in this arrangement. In this arrangement, it is advantageous to use diodes which are not sensitive to signals at frequencies as high as that of the bit rate of the traffic transported on the optical fiber. Supervisory information is thus provided by modulation of one or more of the traffic signals at a substantially lower frequency. If a number of traffic signals are transmitted along the optical fiber and one or more, but not all, are modulated, it will not be possible for the photodiode to determine from which traffic signal or signals the modulation information is derived, as it is not possible for it to distinguish between carrier wavelengths. The response of a photodiode in such an arrangement is discussed further below.
While the use of such low level, low frequency, intensity modulation of the carrier signal does provide advantages over other possible methods previously considered for providing supervisory signals to repeaters, it would be desirable to obtain further improvements. Adjustment of the modulation signal to provide sufficiently reliable information to the individual repeaters, but without an unacceptable increase in the error rate for the traffic signals, presents a problem even when such an approach as indicated above is employed.
U.S. Pat. No. 5,383,046A shows how in a conventional optical supervision method the data signal and the supervisory signal are multiplexed and modulated onto a signal optical carrier wavelength. This leads to considerable problems in transmission and detection as previously mentioned.
Accordingly, the invention provides a method of providing supervisory signals to amplifiers of a fiber optic system, said fiber optic system carrying traffic signals at one or more traffic signal carrier wavelengths from a first terminal station of the fiber optic system, wherein the traffic signals are amplified by said amplifiers so that the traffic signals are maintained in strength along said fiber optic cable system for detection at a second terminal station;
wherein a supervisory signal is provided at a carrier wavelength different from the one or more carrier wavelengths used for the traffic signals, but wherein the fiber optic system is adapted such that the supervisory signal is amplified such that the supervisory signal is maintained in strength along the fiber optic system by the same said amplifiers amplifying the traffic signals, and wherein supervisory information is provided on the supervisory signal by modulation at a frequency substantially lower than that of a traffic bit rate for detection by detectors located at said amplifiers.
In a further aspect, the invention provides a method of providing supervisory signals to amplifiers of a fiber optic system, said fiber optic system carrying traffic signals at one or more traffic signal carrier wavelengths from a first terminal station of the fiber optic system, wherein the traffic signals are amplified by said amplifiers so that the traffic signals are maintained in strength along said fiber optic cable system for detection at a second terminal station;
wherein supervisory signals are provided at two or more carrier wavelengths different from the one or more carrier wavelengths used for the traffic signals, and wherein supervisory information is provided on the supervisory signal by modulation at a frequency substantially lower than that of a traffic bit rate for detection by detectors located at said amplifiers.
Preferably, the supervisory information is provided by modulation at substantially 200% of the supervisory signal. Advantageously, either the supervisory signal carrier wavelength is variable, or two or more fixed supervisory signal carrier wavelengths are provided. Advantageously, a supervisory signal carrier wavelengths is spaced from the traffic signal carrier wavelengths in such a manner that the supervisory signal can be removed by optical filtering at the second terminal station.
The invention further provides a fiber optic system comprising:
a first terminal station;
a second terminal station connected to the first terminal station by fiber optic cable;
means for providing traffic signals at one or more traffic signal carrier wavelengths at said first terminal station;
means for providing a supervisory signal at supervisory signal carrier wavelengths different from said one or more signal carrier wavelengths at said first terminal; and
wherein said fiber optic cable comprises a plurality of amplifiers, the amplifiers of said plurality of amplifiers being adapted to maintain in strength along the fiber optic system both the traffic signals and the supervisory signal, and wherein said amplifiers have associated therewith detectors adapted to detect supervisory information provided by modulation of supervisory signal at a frequency significantly lower than that of a traffic signal bit rate.
Preferably, the detectors are photodiodes, especially silicon diodes. Advantageously, the means for providing supervisory signals comprises a tunable laser source.